1. Field of the Invention
The present invention relates to an operational method for a memory, and more particularly, to a method for programming a multi-level cell (MLC).
2. Description of the Related Art
According to whether data stored in memory devices can be maintained after power is removed, memory devices are categorized into volatile memory and non-volatile memory. Wherein, non-volatile memories can be programmed and erased, and maintain stored data even if power is cut off. Because of these advantages, non-volatile memories have been widely applied in personal computers and electronic equipment.
Generally, a non-volatile memory is composed of many memory cells. Each memory cell is composed of a bottom dielectric layer (tunnel layer), a charge storage layer, a top dielectric layer (charge barrier layer) and a control gate layer which are sequentially formed on a substrate. Moreover, a non-volatile memory cell can store one or more bits in accordance with different materials of the charge storage layer used. For example, if the material of the charge storage layer is a conductive material, a memory cell stores one bit. If the material of the charge storage material is a non-conductive material, two sides of the charge storage layer can be served as two storage positions. Thus, the memory cell stores two bits.
Due to huge computer software programs being applied, capacity of memory should also be increased. The prior-art memory which is adapted to store one or two bits cannot satisfy the requirement described above. Moreover, the requirement for device integrity is increased. Accordingly, how to increase capacity of memory without affecting device integrity becomes an essential topic in this field.